Servo system



United States Patent SERVO SYSTEM Ernest W. Cassaday and Le Roy A. Dettlof, Lafayette, Ind., assignors to Ross Gear and Tool Company, Inc., Lafayette, Ind., a corporation of Indiana Filed Apr. 4, 1960, Ser. No. 19,693

8 Claims. (Cl. 60--52) This invention relates to a hydraulic servo system embodying a reversible hydraulic motor, a power-driven pump, a control pump, and a valve means operated in response .to the pressure differential created by operation of the control pumpto effect the supply of liquid under pressure from the power-driven pump to the motor. More specifically, this invention is directed to such a system in which the control pump, when operating, is always connected in a fluid circuit with the motor with the result that liquid displaced in operation of the motor will pass through the control pump and that the motor can be operated by the control pump in event of failure of the power-driven pump.

When a system of the type referred to embodies a motor subject to relatively heavy loads and a control pump adapted for manual operation, it is necessary'to provide a relatively high mechanical advantage of the control pump over the motor in order to make possible operation of the motor by the control pump when the power-driven pump fails. Under conditions of power operation, such a high mechanical advantage may be objectionable because of the extent and speed of control-pump operation which must accompany operation of the motor at the rate possible under power actuation. Accordingly, it is an object of this invention to produce a system of the type described in which, under power operation, only a portion of the liquid displaced in operation of the motor passes through the control pump.

In one system embodying the invention, the valve means comprises a valve housing having a pair of motor ports connected to the motor, a pair of control ports connected to the control pump, a pressure port for receiving liquid under pressure from the power-driven pump, and a return port. The valve also comprises a movable valve member which can be shifted in opposite directions from a neutral position, toward which it is preferably biased by a preloaded centering means. In the neutral position of the valve member, it serves to interconnect the control ports respectively with the motor ports, thus making it possible for the control pump to be operated to eflfect actuation of the motor. We provide means responsive to the pressure-differential created by the control pump for shifting the movable valve member in one direction or the other from its neutral position, depending upon the direction in which the control pump is operated. When shifted, the movable valve member acts to provide between the pressure and relief ports a fluid circuit which includes the control pump and the motor. Pressure fluid entering such circuit passes through on or another of two restricted orifices, and means responsive to the pressure-drop across each of such orifices is provided to open a bypass permitting some of the liquid displaced in operation of the motor to escape passage through the control pump. The bypass contains a. restricted flow-regulating orifice the size of which, relative to the bypassed orifice, controls the proportion of the displaced liquid which escapes passage through the control pump.

2,974,491 Patented Mar. 14, 1961 Further objects and features of the invention will become apparent from the following more detailed de scription and from the accompanying drawing in which the single figure shows, somewhat diagrammatically, a servo system and, on an enlarged scale, details of a preferred form of valve means.

The system shown in the drawing comprises a motor 10, a control pump 11, a power driven pump 12, and a valve designated in its entirety by the reference numeral 13. While the servo system may be used for any desired purpose, that shown in the drawing is primarily intended as a power steering mechanism for vehicles. To that end, the motor 10 comprises a cylinder slidably re ceiving a piston 16 operatively connected, as :by the rack and pinion shown, to a rock shaft 17 which is adapted for connection to the element or elements being steered. In a power steering system, the control pump 11 would be manually operated by the steering wheel of the vehicle.

The valve 13 comprises a housing 20 provided with a bore 21 which slidably receives a valve member 22 of the spool type. The bore 21 is provided with a central annular groove 23 and two outer grooves 24 and 25 spaced axially in opposite directions from the groove 23. The valve member 22 has a central land 26, intermediate lands 2'7 and 23, and end lands 29 and 29. In the neutral position of the valve shown, the center land 26 occludes the groove 23, and the intermediate lands 27 and 28, which have substantially the same width as the grooves 24 and 25, overlap only the outer edges of such grooves permitting communication of the groove 24 with the spaces between the lands 26 and 27 and of the groove 25 with the space between the lands 26 and 28.

The valve body 20 is provided with a pair of control ports 30 and 31 connected by conduits 32 and 33 with opposite sides of the control pump l. The valve body also has a pair of motor ports 35 and 36 connected respectively by conduits 37 and 38 with opposite ends of the motor-cylinder 15. A pressure port 40 receives liquid discharged from the power-driven pump 12, while a return port 41 is connected to a conduit 42 discharging into a sump 43 from which the pump 12 draws liquid.

The control ports 3%) and 31 are provided respectively with inner portions 45 and 46 of reduced diameter which communicate at their inner ends with the bore 21 on opposite sides or" the land 26 and which provide upwardly presented shoulders against which valve members 47 and 43, respectively, slidable axially of the ports 30 and 31 are seated by springs 47' and 48'. Extending axially through each of such valve members is a passage including a rtstricted orifice 50. In the plane of the ports 30 and 31 and outwardly from the shoulders previously referred to the valve housing 26 is provided with a passage 52 intersecting both of the ports 30 and 31 and extending outwardly therebeyond. One end of the passage 52 is connected through a check valve 53 with the reduceddiameter portion 45 of the port 39, the check valve opening away from the passage 52 through a valve seat 54. A similar check valve 55 having an associated valve seat 56 interconnects the opposite end of the passage 52 with the inner, reduced-diameter portion 46 of the port 31. Within the transverse extent of the passage 52, the valve members 47 and 48 are of such diameter that they do not prevent the flow of liquid through that passage.

A passage 6% formed in the valve housing 20 pr0- vides communication between the pressure port 40 and the center groove 23. A second passage er in the valve body extends from the return port 41 to the bore 21, communicating therevn'th at a point between the lands 27 and 29. The space between the lands 27 and 2-9 communicates with the space between the lands 2-8 and 29 through an axially extending passage 62 in the valve member 22. In the neutral position of the valve, the

passage 62 communicates with the groove 23 through one or more radial passages 62 formed in the valve: land 2.6.

Desirably, the passage 61 contains a load valve 63 opening outwardly, or toward the return port 41. For a pu'rpose which will hereinafter be described, the valve 63"may have extending through it an axial passage provided with a check valve 64- opening inwardly, or toward the bore 21. Above the valve 63, the passage 61. communicates with the passage 60 through a check valve 65 opening toward the latter passage.

To provide for shifting of the valve member 22 in response to operation of the control pump 11, the ends of the bore 21 are closed by closures 66 and 67 which, incooperation with the opposite ends of the valve member 22, form chambers 68 and 69. The chambers 68 and 69 are connected respeotively'by conduits 70 and 71 with the conduits 32 and 33 leading from the control pump 11 whereby the valve member 22 will be movable in the bore 21 under the influence of the pressure-differential resulting from operation of the control pump 11.

Preferably, the valve 13 includes a resilient, preloaded centering means acting to urge the valve member 22 toward its neutral position when it is displaced therefrom. As shown, the valve member 22 is provided with a headed eitension 75 projecting into the chamber 69, and'o'n such extension there is slidable a washer 76 which is urged toward engagement with the adjacent end of the valve 22 and with the adjacent face of the housing by inner and outer compression springs 77 and 78,

the spring 77 reacting against the head of the extension 75 and'the spring 78 reacting against the closure 67. In the neutral position of the valve member 22, the Washer 76,engages both the adjacent end of the valve member and the face of the housing 20, and the valve member can be displaced from neutral position only by the application of an effort sufficient to overcome one or the other of the springs 77 and 78. Desirably, both of such springs are stressed in the neutral position of the valve member 22 so that such member will remain in neutral position until the differential pressure created by the control pump 11 becomes sutficient to overcome the preloading resulting from the stress in the spring, 77 or 78, opposing such differential pressure.

With the power-driven pump operating and the valve member 22 in the neutral or centered position shown in the drawing, liquid delivered from the pump 12 enters the pressure port 40 and passes, by way of the passages 60, 62', 62, and 61 to the return port 41, from which it returns to the sump 43 through conduit 42. If the control pump 11 is operated,'say to draw liquid from the conduit 33'and deliver it to the conduit 32, the liquid delivered to the conduit 32 will flow by way of control port through the orifice 50 and the valve member 47 into the bore" 21 between the lands 26 and 27 and will then pass by way of the motor port and conduit 37 to the lefthand end of the cylinder 15, urging the piston 16 therein to the right. Liquid displaced in the rightward movement of the piston 16 will flow by way of the conduit 38 and motor port 36 into the bore 21 between the lands 26 and 28, and will then flow through the valve member 48, control port 31, and conduit 33 to the pump 11. The pressure-differential created by operation of the pump 11 will be transmitted to the chambers 68 and 69 by way of the conduits 70 and 71 and will urge the valve member 22 toward the left. If the load on the motor 10 is low enough, the control pump 11 may be operated without creating a differential pressure suificient to overcome the effect of the spring'78, and the valve will remain centered. However, if the load on the motor 10 is such that the pressure differential created by operation of the pump 11 is sufficient to overcome the spring 78', the valve member 22 will be displaced toward the left to' open communication between the center groove 23 and the inner end of the port 31 and to block comrnnnica I tion between that port and the groove 25. In such shitting rnjdven'ientoftli'e vatve the inner end of the port 30 will remain connected to the motor port 35 and the return port 41 will remain connected to the bore 21 between the lands 27 and 29. As a result of the uncovering of the groove 23, pressure liquid delivered from the pump 12 will enter the port-portion 46-,"from which it can escape only through the axial passage in valve 48, since such valve is initially seated and-the check valve prevents direct flow into passage 52. Accordingly, the pressure liquid will flow through the orifice 50 in the valve 48, and, by way of conduit 33, control pump 11, and conduit 32 to the control port 39, whence it will pass through the seated valve 47, motor port 35 and conduit 37 to the left-hand end of the cylinder 15. However, the pressure drop across the orifice 50 in the valve 48 will raise such valve to provide communication between the inner portportion 46 and the passage 52, thus permitting some of the pressure liquid. supplied to the port portion 46 to flow through passage 52 and'past check valve 53 to the inner port portion 45, where it joins liquid delivered by the control pump to conduit 32 and flows with such liquid to the left-hand end'ot the motor cylinder 15. It will thus be seen that a part of the pressure liquid delivered from the. pump 12 will flow to the left-hand end of the cylinder 15 by way of the control pump 11 while the remainder will flowto the left-hand end of the motor cylinder without passing through the control pump 11. It is contemplated that the springs 47 and 48 will be relatively weak so that when the valve 48 opens it will open widely and. permit liquid t o enter the passage 52 without significant pressure-drop. In consequence, the division otfthe pressure liquid between those two paths of flow will depend upon the relative areas of the orifice 50 in the valve'48' and the orifice in the valve seat 54.

liquid displacedlfrom the right-hand end of the motor cylinder 15 will flow by. way of conduit 38, motor port 36 and pas s'ages 62 and.61 to the return port 41, whence it will flowf through conduit 42 to the sump, 43. The check valve 63 in'the 'p as sage 61 is spring loaded to provide a minimum positive pressure in the return line, thus inhibiting the creation ofa vacuum such as might be created inthejs'ystemlshould the control pump be operat'ed too ..'r'apidly.

Ifthecontrol pump 11 is operated in the opposite direction tov draw liquid through the conduit 32 and deliveritftoihe conduit 33, theliquid so delivered may pass" through the orifice 50 .and valve member 48 into the ,bor'e'21 between the lands 26 and 28, and thence by way of the motor port 36, and conduit 38 to the righthand-end ofthe motor cylinder, while liquid displaced from the left-hand'end of the motor cylinder flows by way of'c'onduit 37, motorport 35, control port 30 and conduit 32 to the control pump. Pressure differential created by operationof the control pump is transmitted to the chambers 68 and 69, through the conduits 70 and 71" and serves to urgethe valve member 22 to the right against the efiort exerted by the spring 77. If the pressure differential is suflicient to overcome the spring 77, the valve member 22 is displaced to the right to block communication between the pressurepassage and the passage 63, and between control port 30 and motor port 35 and to open communication between the pressure passage 60., -and'the inner end,45 of port 30. Since the valve 47 is initially seated and since the check valve 53 prevents direct flow from port portion 45 to passage 52, the pressureliquid will be compelled to flow through the valve 47, conduit 32, control pump 11, conduit 33 and control port 31 to the motor port 36, and thence by way ofconduit 38 to the'right-hand end of the cyliner. The pressure'drop, across the orifice 50min the valve member 47 elevates suchivalve. member and permits a portion of the ,p'ress'nre, liquid supplied to the inner end 45 .of the Pen- M 'Qw-t t h re s sefl nd P sk ve 55 into the bore 21 between the lands 26 and 28, where 5 it joins the liquid delivered to port 31 from the control pump and flows with such liquid by way of motor port 36 and conduit 38 to the right-hand end of the motor cylinder. As before, the proportion of pressure liquid passed through the control pump will depend on the relative areas of the orifices 50 and 54. Liquid displaced from the left-hand end of the cylinder flows by way of conduit 37 and motor port 35 into the bore 21 between the lands 27 and 29, whence it is returned to the sump 43 through passage 61 and return port 41.

If the power-driven pump 12 should fail for any reason, the system described may be operated entirely by the control pump 11. So long as the load on the motor is low enough to permit the motor to respond to a pressure diiferential insuificient to shift the valve member 22 from neutral position, the control ports 30 and 31 will be connected respectively to the motor ports 35 and 36, and liquid can be transferred through the thus connected ports directly between the control pump 11 and the motor. Under heavier loads, the pressure differential created by the control pump 11 may become sufiicient to cause the valve member 22 to shift from neutral position. If such shift is to the right, control port 31 will remain connected to motor port 36, so that the liquid delivered from control pump 11 into the conduit 33 will pass by way of ports 31 and 36 and conduit 38 to the right-hand end of the motor cylinder 15. Liquid displaced from the left-hand end of the motor cylinder will pass through conduit 37 and port 35 into valve groove 24; but since the rightward shifting of the valve member 22 has blocked direct communication between the valve groove 24 and control port 30, the returning liquid will be delivered to the bore 21 between the valve lands 27 and 29 and will pass to port 30 by way of the passage 61, check valve 65, passage 60, and groove 23. If the control pump is operated in the opposite direction to create suificient ditierential pressure to move the valve member 22 to the left, the liquid delivered by the con trol pump will pass through conduit 32, ports 30 and 35, and conduit 37 to the left-hand end of the motor cylinder 15, while the liquid displaced from the right-hand end of such cylinder will be delivered through conduit 38 and motor port 36 to the bore 22 between the valve lands 28 and 29, whence it will pass through passages 62 and 61, check valve 65, and passage 60 to return to the pump by way of port 30 and conduit 32.

It will be noted that in both directions of operation under the conditions just described the passage 61 will be under pump suction. The check valve 63, which opens toward the passage 61 from the return line 42, serves to prevent creation of a vacuum in the system; because should any tendency to create a vacuum arise, liquid will be drawn into the system past such check valve. To maintain liquid in the return line 42 for that purpose, such line may extend to a point below the liquid level in the sump 43.

In either direction of operation of control pump 11, when the power driven pump 12 is not operating, the returning fluid will pass upwardly through one or the other of the valves 47 and 48 and will create across the orifice 50 in such valve a pressure-drop tending to raise it, but any such raising of either valve will have no significant consequence. For example, if the returning fluid passes upwardly through valve 47 and creates at the orifice 50 therein a pressure drop sufiicient to cause lifting of the palve, the only consequence would be the application of pump-suction from the control pump 11 to the passage 52; but, since the check valves 53 and 55 close toward the passage 52 and since the valve 48 will be seated, no liquid can enter passage 52 as the result of such suction.

It will be apparent from the above description that when the motor is operated entirely by the control pump all liquid displaced will pass through the control pump 11 and that the rate at which the motor responds to operation of the control pump will depend upon the eff ec tive mechanical advantage between the control pump and the motor, By properly coordinating the displacement of the control pump and the cross-sectional area of the motor cylinder, that mechanical advantage can be made as great as may be necessary to enable the control pump to operate the motor. If the servo system is used as a power steering gear, increasing the mechanical advantage would inevitably entail correspondingly increasing the extent of steering-wheel rotation required to produce a given deflection of the steered element or elements. Under power operation, where the mechanical advantage is not needed, a high ratio of steering-wheel rotation to steered-element deflection might well be objectionable, and can be avoided by the use of our invention. For example, if the area of each orifice 50 is one fourth that of each of the valve seats 54 and 56, the ratio of steering-wheel rotation to steered-element deflection under power operation will be one-fifth that obtaining under manual operation, for in the latter case only one-fifth of the liquid displaced in operation of the motor will pass through the control pump.

It is to be understood that the particular embodiment of the invention illustrated and above described is but one of many forms our invention may take.

We claim as our invention:

1. In a hydraulic servo system having a reversible hydraulic motor, a power driven pump, a reversible control pump, and a control valve for controlling the flow of liquid between said pumps and said motor, said valve having a housing provided with first and second motor ports connected to the motor, first and second control ports connected to said control pump, a pressure port receiving liquid discharged from said power-driven pump, and a return port, said valve also having a valve member movable in opposite directions from a neutral posi-' tion to control the interconnection of said ports, said valve member when in neutral position serving to interconnect said first and second control ports respectively with said first and second motor ports and to block communication of the motor ports and control ports with said pressure and return ports whereby to permit the control pump to operate the motor, means responsive to the pressure differential resulting from operation of said control pump for urging said movable valve member from neutral position, resilient centering means opposing displacement of said valve member from neutral position, said centering means being preloaded to maintain the valve member in neutral position until said pressure differential reaches or exceeds a predetermined value, said valve member when in displaced position interconnecting said ports to provide between said pressure and relief ports a fluid circuit including the control pump and the motor whereby liquid displaced in operation of the motor will pass through the control pump, said valve including passage-defining means including a restricted orifice for conveying liquid to and from each of said motor ports, said body having first and second bypass passages for conveying pressure liquid entering said pressure port to said first and second motor ports respectively, each of said bypass passages including a restricted orifice and a check valve opening toward the associated motor port, and normally closed valve means for controlling flow of pressure liquid through each of said bypasses, each of said valve means being responsive to the pressure drop across the orifice in the associated passagedefining means to open its associated bypass when liquid flows through such orifice to the associated control port.

2. In a hydraulic servo system having a reversible hydraulic motor, a power driven pump, a reversible control pump, and a control valve for controlling the How of liquid between said pumps and said motor, said valve having a housing provided with first and second motor ports connected to the motor, first and second control ports connected to said control pump, a pressure port receiving liquid discharged from said power-driven pump, and a return port, said valve also having a valve membermovable in opposite directions from a neutral position to control the interconnection of said ports, said valve member when in neutral position serving to interconnect said first and second control ports respectively with said first and second motor ports and to block communication of the motor ports and control ports with said pressure and return ports whereby to permit the control pump to operate the motor, means responsive to the pressure differential resulting from operation of saidcontrol pump for urging said movable valve member from neutral position, said valve member when in displaced position interconnecting said ports to provide between said pressure and relief ports afiuid circuit including the control pump and the motor whereby liquid displaced in operation of the motor will pass through the control pump, said valve including passage-defining means including a restricted orifice for conveying liquid to and from each of saidmotor ports, said body havingfirst and secondzbypass passages for conveying pressure liquid entering said pressure port to said first and second motor ports respectively, each of said bypass passages including a retricted orifice and a check valve opening toward the associated motor port, and normally closed valve means for controlling flow of pressure liquid through each of said bypasses, each of said valve means being responsive to the pressure drop across the orifice in the associated passage-defining means to open its associated bypass when liquid flows through such orifice to the. associated control port.

3. A servo system as set forth in' claim 2 with the addition that each of-said valve means includes a valve member movable away from its associated control port to closed position, said valve members having longitudinal passages respectively containing said first mentioned orifices.

4. A hydraulic servo system comprising a motor, a

8 1 control pump, a power driven pump, conduits providing a liquid circuit including said control pump and motor; and-a control'valve controlling the flow of liquid between saidpumps and said motor, said control valve including ,a' movable valve member having a neutral position: in

whichlit sorvesto exclude said power driven pump' from saidl circuit, said valve member being displaceable from neutral position to connect said power driven pumpinto said circuit in series with said control pump, and means operating automatically in response to the admission of 'liquid' from the power-driven pump into said circuit for bypassing a predetermined portion of the flowing liquid around the control pump. 1

5 A servo system as set-forth in claim 4 with the addition of' means responsive to the pressure differential created by operation of said control pump for displacing said valve member from neutral-position.

6.- A servo system as set forth in claim 5 with the addition of preloaded resilient centering means opposing displacement of said valve member from neutral position;

7. A servo system as set forth in claim 4 with the addition that said last named means includes means providing a restricted orifice for the passage of liquid flowing through thecontrol pump, a bypass passage around the control pump, and' a normally closed valve element in said bypassage openable in response to the pressure drop across'said orifice.

8. A servo system as set forth in claim 7 with the addition that said orifice is located in a passage extending through said valve element.

References Cited in the file of this patent UNITED" STATES PATENTS 2,836,960 Wittren June 3, 1958 

